In common radar devices, when displaying a radar image, a target object is displayed in a color according to an intensity of a received echo. In such a radar device, an operator typically looks at the radar image for several scans and determines by his/her experiences whether a target object currently displayed on a screen will be an obstacle for a ship concerned (that is, whether there is any risk of a collision).
However, if the operator is inexperienced for reading such a radar image, it is difficult to determine whether the target object currently displayed will be an obstacle. Even if the operator is well experienced for reading such a radar image, he/she cannot determine a collision risk level unless he/she watches the screen at least for several scans.
For this point of view, JP2003-48595(A) discloses an other ship display device which is configured so that other ships with high degrees of collision risk are displayed by high visibility and other ships with low degrees of collision risk are displayed by low visibility. Thus, because the disclosure displays target objects with high degrees of collision risk by high visibility, the display method is particularly suitable for the purpose of collision avoidance.
Further, JP2003-48595(A) uses AIS (Automatic Identification System) or ARPA (Automatic Radar Plotting Aids) as an instrument for detecting information on other ships.
ARPA uses a technique for detecting moving vectors (moving speed and direction) of a target object from a transition of past radar images. However, because ARPA typically suffers a large signal processing load, the number of the target objects which can be acquired and tracked is limited. In order to detect the moving information on the target object based on the radar images for past several scans, there arises a problem of taking time for detecting that the target object changed the moving speed and the direction. Further, if irregular echoes, such as sea surface reflections, appeared near the target object, there also arises a problem that the captured image of the target object is blended with the sea surface reflection image or the like and, thus the target object will be lost.
On the other hand, AIS is a technique of periodically transmitting positional information, traveling information and the like on a ship concerned by wireless communication and receiving such information from other ships to extract required information therefrom. However, if other ships do not carry AIS, the information on the other ships cannot be detected. Further, because a repetition of communication is limited, the information on the other ships cannot be acquired quickly.
As described above, it is difficult for the configuration of JP2003-48595(A) to certainly grasp the target objects with high degrees of collision risk to display them in real time.